結晶技術對複晶矽薄膜電晶體元件 特性及缺陷分布之影響

Abstract

為了提升複晶矽薄膜電晶體的特性，高品質的複晶矽薄膜是關鍵，而如何開發低成本低溫的結晶技術，是整個低溫複晶矽面板技術再突破的首要之務；在本論文內，我們比較四種結晶技術的薄膜品質，包含量產線之準分子雷射退火、固態晶體雷射退火、固相退火及飛秒雷射退火四種結晶技術，並分析四種技術製作之複晶矽薄膜電晶體的元件特性。其中，前三種結晶技術都是靠熱能結晶的技術，而飛秒雷射退火則是新型非熱結晶技術，具有低雷射能量即可結晶之特性，是我們此篇論文討論的重點技術。 飛秒雷射退火是利用長波長（800奈米）、低能量密度（約45 mJ / cm2）之飛秒雷射掃瞄非晶矽薄膜樣品以進行結晶，調整不同能量密度，可以變化樣品的結晶性以及平坦度，本篇論文中利用SEM、AFM、以及拉曼分析等技術，來分析此飛秒雷射退火之最佳製程條件。同時，也製作了利用飛秒雷射退火結晶之複晶矽薄膜電晶體，分析其元件特性和薄膜品質的相依性，在最佳結晶條件下，飛秒雷射退火再結晶元件擁有如低的臨界電壓約2.0 V、次臨界斜率趨近於0.8 V / dec.、開關電流比約2.33 x 107以及場效載子傳輸率約62 cm2 / Vs，首次展示了非熱再結晶技術在製作複晶矽薄膜電晶體上的可行性，同時，和同樣製程下製作的固相退火元件相比較，飛秒雷射退火之元件可以有較佳的特性。 最後我們針對不同的結晶技術去分析薄膜的缺陷分布情形，從次臨界斜率去萃取斷鍵態(deep state)密度，並利用I-V變溫量測的活化能數據去萃取缺陷的能態分佈，觀察扭曲鍵態(tail state)密度和場效載子傳輸率的關連性；最後，我們從C-V量測觀察並討論缺陷態的頻率響應，並和元件的電流電壓特性做比對及分析。

In order to improve the characteristics of poly-Silicon thin-film transistors (TFTs), the high quality of poly-silicon film is the key issue. And how to develop a low cost and low temperature crystallization technology is the prime mission of the whole LTPS panel technology. In this thesis, we compared the film qualities under four different crystallization technologies, including the mass production excimer laser annealing (ELA), solid state laser (SSL) annealing, solid-phase crystallization (SPC) and femtosecond laser annealing (FLA). Besides, we also analyzed the device performance fabricated by the four kinds of crystallization technologies. Since the former three crystallization technologies are crystallized by thermal energy, and FLA is a new, non-thermal crystallization technology which has the characteristics of crystallization formed by a lower laser energy density. Therefore, it is the main technology we discuss in the thesis. Amorphous-silicon film crystallized by line-scan near-infrared femtosecond laser annealing, which is long wavelength (800 nm), low energy density (about 45 mJ/cm2). As we adjusted different laser energy density, we could change the crystallinity and the roughness of the sample. In this thesis, we introduced the SEM, AFM, and Raman analysis to find out the best process condition of FLA. Furthermore, we also fabricated poly-Silicon TFTs by FLA crystallization and analyzed the dependence of the film quality and the device performance. FLA poly-Silicon TFTs demonstrated a lower threshold voltage of 2.0 V, a steeper subthreshold swing of 0.8 V / dec., a higher ON/OFF current ratio of 2.33 x 107 and a higher field-effect mobility of 62 cm2 / Vs, at the optimal crystallization condition. For the first time, it shows the feasibility of fabricating poly-Silicon TFTs by a non-thermal crystallization technology. Moreover, the device crystallized by FLA has better performance compared to that crystallized by SPC under the same fabrication condition. At last, we focused on the defect distribution analysis of various crystallization technologies. We extracted the deep state density from subthreshold swing and the bandgap defect distribution from thermal activation of conduction current using I-V measurement. Finding out the relationship of the tail state density versus field-effect mobility and the behavior of each other were also demonstrated. In addition, we observed the correlation of the tail state density and the field-effect mobility. Then we discussed the frequency response of the defect state by C-V measurement and made a comparison of I-V characteristics of the device.